There is a great need for metal alloys having high strength and good ductility which can withstand adverse environments, such as corrosion and carburization, at increasingly higher temperatures and pressures. The upper operating temperature of conventional heat resistant alloys is limited to the temperature at which second phase particles are substantially dissolved in the matrix or become severely coarsened. Above this limiting temperature, the alloys no longer exhibit useful strength. One class of alloys which is exceptionally promising for such uses are dispersion strengthened alloys obtained by mechanical alloying techniques. These dispersion strengthened alloys, especially the oxide dispersion strengthened alloys, are a class of materials containing a substantially homogeneous dispersion of fine inert particles, which alloys can exhibit useful strength up to temperatures approaching the melting point of the alloy material.
The primary requirement of any technique used to produce dispersion strengthened metallic materials is to create a homogeneous dispersion of a second (or hard) phase which has the following characteristics.
(i) small particle size (&lt;50 nm), preferably oxide particles; PA1 (ii) low interparticle spacing (&lt;200 nm); PA1 (iii) chemically stable second phase, [The negative free energy of formation should be as large as possible. The second phase should not exhibit any phase transformation within the operation range of the alloy]; PA1 (iv) the second phase should be substantially insoluble in the metallic matrix.
Dispersion strengthened alloys are generally produced by conventional mechanical alloying methods wherein a mixture of metal powder and second, or hard phase particles are intensively dry milled in a high energy mill, such as the Szeguari attritor. Such a process is taught in U.S. Pat. No. 3,591,362 for producing oxide dispersion strengthened alloys, which patent is incorporated herein by reference. The high energy milling causes repeated welding and fracturing of the metallic phase, which is accompanied by refinement and dispersion of the hard phase particles. The resulting composite powder particles are generally comprised of a substantially homogeneous mixture of the metallic components and an adequate dispersion of the second, or hard phase. The bulk material is then obtained by hot or cold compaction and extrusion to final shape.
One reason for the lack of general adoption of commercial dispersion strengthened alloys, for example oxide dispersion strengthened alloys, by industry has been the lack of technically and economically suitable techniques for obtaining a uniform dispersion of fine oxide particles in complex metal matrices that are free of microstructural defects and that can be shaped into desirable forms, such as tubulars. Although research and development on oxide dispersion strengthened material have continued over the last two decades, the material has failed to reach its full commercial potential. This is because prior to the present invention, development of microstructure during processing which would permit the control of grain size and grain shape in the alloy product was not understood. Furthermore, there was no explanation of the formation of intrinsic microstructural defects introduced during processing, such as oxide stringers, boundary cavities, and porosity.
Oxide stringers consist of elongated patches of oxides of the constituent metallic elements. These stringers act as planes of weakness across their length as well as inhibiting the control of grain size and grain shape during subsequent recrystallization. Porosity, which includes grain boundary cavities, is detrimental to dispersion strengthened alloys because it adversely affects yield strength, tensile strength, ductibility, and creep rupture strength.
Consequently, there is a need in the art for methods of producing dispersion strengthened alloys free of such defects as oxide stringers and porosity.